Prevent new lines written by write_str from breaking source maps

[thecrypticace]

Right now putting license comments in a file causes invalid source maps to be generated.

For example, running this file through lightning with source maps will produce incorrect generated locations:

/*! foo 
 foo 
 foo 
 foo */
/*! bar */
.flex {
  display: flex;
}
.hidden {
  display: none;
}

Here's a screenshot of the source map visualizer for the generated CSS:

This happens because Lightning computes the column offset to be the length of the comment plus the new line character at the end and this happens for each license comment in the file. Additionally, license comments are one of the few things that basically preserve the content as is including new lines. So if the license comment itself contains new lines than the generated mappings can be pretty far off — as the screenshot above shows.

This PR fixes this by tracking new lines and resetting the appropriate state when printing the CSS in write_str.